Low-frequency ultrasound imaging systems are very commonly used in diagnostic medicine, and they have been used for over 50 years. New high-frequency ultrasound imaging technology offers dramatic improvements in image resolution compared to these conventional low-frequency systems. Notwithstanding the increased performance that is possible with high-frequency ultrasound imaging, there are many technical barriers preventing its widespread use. Some of these barriers may be addressed by using array-based systems for high-frequency ultrasound imaging, but fabricating transducer arrays and the associated beamformers is more difficult for high-frequency systems since much smaller dimensions are involved (e.g., the element to element pitch of the transducer).
If an array is fabricated without having sufficiently small dimensions, large image artifacts result called grating lobes. Another unsolved problem of existing systems is that there is no simple and effective way to suppress grating lobes for ultrasound imaging systems that have array transducers with a large element-to-element pitch. One technique that has been proposed for suppressing the grating lobes is described in J. Camacho, M. Parrilla, and C. Fritsch, “Phase Coherence Imaging,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, Vol. 56, No. 5, pp. 958-974, 2009. This technique, called “phased coherence imaging,” suppresses grating lobes using phase coherence correction factor receive beamforming and synthetic aperture transmit beamforming.
Synthetic aperture beamforming is not suitable for use in high-frequency ultrasound imaging where small vibrations can create phase shifts in the received signals. Although synthetic aperture beamforming can produce high frame rates for generating full 2D images, all of the elements need to be pulsed individually before the beamforming delays are inserted. This means that this beamforming technique is susceptible to image distortion due to the large amount of time expired during the acquisition of the pre-beamformed signals. This image distortion is avoided however when implementing transmit focal-zone beamforming. Although only one A-scan line can be collected per transmit event, image distortion due to small motion artifacts is avoided due to the small amount of time expired between beamforming events. Unfortunately, for phase coherence imaging, transmit beamforming creates very long pulses in the grating lobe region which, upon returning to the array elements, create very long narrow band receive pulses. Consequently, when phase coherence correction factors are calculated from the received echoes in the same temporal region as the main lobe, there are no longer any random phases present since all of the long grating lobe echoes now overlap and for a certain time duration, are virtually all in-phase.
Thus, a need exists in the art for improved methods that effectively shorten the grating lobe signals in received ultrasound echoes, thereby enabling improved signal processing and suppression of grating lobes.